Memory devices are used to store data or programs for computers and other electronic devices. Common types of memory include optical storage, such as CDs and DVDs; magnetic storage, such as hard disc drives; and semiconductor memory. Semiconductor memory is a type of memory that is implemented on semiconductor-based integrated circuits. Semiconductor memory can be either volatile or nonvolatile. Volatile memory refers to memory that loses its data when there is no active power source. Nonvolatile memory, on the other hand, does not lose its data when there is no active source. Thus, nonvolatile memory is often used for storage solutions since it does not require an active power source.
While there are many different types of nonvolatile semiconductor memory, one of the most widely used types is flash memory. Flash memory is often found in devices such as mobile phones, MP3 players, USB drives, digital cameras, etc.
Flash memory may be either NOR flash memory or NAND flash memory. NAND flash memory generally has greater storage density, lower cost per bit, and greater endurance than NOR flash memory. Due to the demand for larger data storage capacities, new technologies are frequently being implemented to further shrink the size of a NAND memory cell, thus increasing the storage density, in order to improve this capacity.
NAND flash memory is typically made up of two-dimensional cell array structures. However, the size of two-dimensional NAND flash memory cells is expected to reach its technological limit, estimated to be around 20 nm. Thus, there have been several proposals for a three-dimensional cell array structure for NAND flash memory to allow for further reduction in size. Using a 3D structure, it is estimated that NAND flash memory can achieve higher data storage capacity and effectively lower fabrication cost without relying on advances in lithography technology.
3D structured NAND flash memory uses tunneling for write operations and hole injection for erase operations. Tunneling occurs when a charge is applied to a semiconductor, enabling electrons to move across an insulating layer to provide a charge to a charge storage structure in the semiconductor. Hole injection refers to the opposite effect, where a charge is applied to a semiconductor to provide holes for electrons to move across the insulating layer in order to remove a charge from the charge storage structure. Several candidates for 3D structured NAND flash memory have been proposed, such as P-BiCS (Pipe-shaped Bit Cost Scalable), TCAT (Terabit Cell Array Transistor), VSAT (Vertical Stacked Array Transistor), and VG (Vertical Gate). In a TCAT structure, for example, holes for hole injection may be provided by a hole reservoir in the substrate. For P-BiCS, however, there is no hole reservoir for hole injection.
Thus, in a BiCS-type structure, such as P-BiCS, the generation of holes by gate induced drain leakage (GIDL) is required to perform an erase operation on a vertical NAND string. This involves applying a voltage to a string selection transistor in order to generate a GIDL current. The high electric fields near the string selection transistor junction result in GIDL current generation such that holes get injected into a vertical channel of the string. This sets the potential of the vertical channel of the string such that a negative potential can be applied to any cell gate to erase a cell in the string.
GIDL, however, requires large electric fields within the gate-drain overlap region of a device. These fields produce hot carriers that can lead to gate dielectric degradation of the selection transistor. In addition, integration process control (doping profile control) to form GIDL is difficult.